The striking growth recently in mobile electronic devices, communications devices and the like has been accompanied by a strong desire, from the standpoint of economics and the trend toward smaller and lighter weight devices, for secondary batteries having a high energy density.
Hitherto known approaches for increasing the capacity of this type of secondary battery include, for example, methods which use oxides of vanadium, silicon, boron, zirconium, tin or the like, and complex oxides thereof, as the negative electrode material (see, for example, Patent Documents 1 and 2); methods which use melt-quenched metal oxides as the negative electrode material (see, for example, Patent Document 3), methods which use silicon oxide as the negative electrode material (see, for example, Patent Document 4), and methods which use Si2N2O and Ge2N2O as the negative electrode material (see, for example, Patent Document 5).
To impart the negative electrode material with electrical conductivity, there are also methods that involve the mechanical alloying of SiO with graphite followed by carbonization (see, for example, Patent Document 6), methods wherein the surface of silicon particles is coated with a carbon layer by chemical vapor deposition (see, for example, Patent Document 7), and methods wherein silicon oxide particles are coated on the surface with a carbon layer by chemical vapor deposition (see, for example, Patent Document 8).
However, although these conventional methods do increase the charge-discharge capacity and raise the energy density, the cycle characteristics are inadequate or the battery continues to fall short in terms of the properties desired by the market and so is not always satisfactory. Hence, there has existed a desire for further improvement in the energy density.
In particular, in Patent Document 4, a high-capacity electrode is obtained by using silicon oxide as the negative electrode material for a lithium ion secondary battery. However, to the inventor's knowledge, drawbacks include a large irreversible capacity during initial charge-discharge and cycle properties that have not achieved a practical level. Hence, there remains room for improvement.
As for the art that has imparted electrical conductivity to the negative electrode material, in Patent Document 6, on account of solid-to-solid fusion, a uniform carbon coat does not form, resulting in a poor electrical conductivity.
In the method of Patent Document 7, although a uniform carbon coat can be formed, because silicon is used as the negative electrode material, the swelling and shrinkage during lithium ion intercalation and deintercalation are so large that the coat is unable as a result to withstand practical use, and so the cycle characteristics decrease. To prevent this from happening, a limit must thus be set to the amount of charging. In the method of Patent Document 8, although an improvement in the cycle characteristics has been confirmed, owing to the precipitation of fine silicon crystals, the structure of the carbon coating and inadequate fusion with the substrate, as the number of charge-discharge cycles increases, the capacity gradually declines and, after a given number of cycles, drops off sharply. Hence, this art is not yet ready for use in secondary batteries. In light of the above, the development of a negative electrode material for lithium ion secondary batteries which retains the advantages of high battery capacity and low coefficient of cubical expansion inherent to silicon oxide-based materials, yet also has a high initial charge-discharge efficiency and excellent cycle characteristics, and of a method for manufacturing such a material, has been awaited.